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48                                CHEMICAL ENGINEERING
should have bypasses. Small valves (especially those of brass) have threaded tops: larger valves have bolted tops. Valves are obtainable in "standard/' "extra heavy" and some special weights.
Size of Pipes.—Velocities in pipes were formerly 6,000 ft. per minute for live and 4,000 ft. for exhaust steam. With superheated steam velocities up to 15,000 ft. are regularly used. Very large pipes should be avoided. The size should be calculated for the allowable pressure drop:
„_„
Where, w — Weight of steam carried, pounds per minute,
p = Pressure drop from beginning to end of line, pounds per sqnare inch, D = Weight of steam per cubic foot at the mean pressure (see steam
table, p. 49),
di = Actual internal diameter of pipe, inches, and L = Length of pipe, feet.
For each entrance, elbow or valve, there is additional resistance to flow about equivalent to that set up by a straight pipe length of about 60 diameters.
Heat Loss.—Bare pipe emits about 3 B.t.u. per hour per square foot for each degree difference of temperatures of steam in the pipe and air surrounding it. Drafts of air greatly increase this. Commercial coverings reduce this loss by 70 to 90 per cent. In general all piping containing heat which is still to be used is worth covering. It usually pays to cover flanges, valve bodies, etc., on high pressure steam lines: but the covering should be applied only after tightness has been demonstrated and should be so applied as to interfere as little as may be with repairs. Sectional covering (jacketed or banded) is used in preference to plastic where possible. Durability of covering should be considered as well as insulating properties and cost.
Outdoor steam pipes may be designed for low pressure drop and low heat loss. They should never be buried in the ground. For large installations, tunnels give good access and minimum loss, but are very expensive ($20 upward per running foot). Overhead lines carried on poles or towers should have waterproofing outside the insulation.
Specialties.—The back-pressure valve is a low-pressure safety valve usually applied to vent surplus steam from a low-pressure exhaust or heating system. The automatic relief valve is of similar purpose but more elaborate design, applied on vacuum lines to give an emergency vent should the vacuum fail. Pressure regulating or reducing valves are used to maintain a steady low pressure for heating or process work less than the prevailing high pressure of the plant. When used on heating systems, the low pressure for which they are set should be less than that at which the back-pressure valve operates. Foot valves are used at the base of vertical suction pipes to retain some water when the pump is shut down. A strainer should be incorporated here. Boiler blowoff valves should be installed in pairs and should be rugged, with renewable seats. Automatic stop valves, placed next to boiler nozzles, stop the flow of steam should a pipe burst. Nor^return valves similarly stop the return flow in case of rupture of a boiler tube. Check valves are of the swing (flapper) and globe (vertical spindle) types. They are used on the boiler-feed line. Steam traps drain off water